This proposal is primarily concerned with the electron microscopic examination of the morphology, as well as the temporal and spatial distribution, of the synaptonemal complex and of the recombination nodule in the oocytes of Drosophila melanogaster. Recombination nodules are structures associated with the synaptonemal complex and the synapsed bivalents at pachytene; the numbers and distributions of nodules correspond quite closely with the numbers and distributions of exchange events, suggesting that nodules indicate the sites at which exchange is occurring (Carpenter 1975b). One objective of the proposed research is to continue to examine the role of the recombination nodule in the exchange process; this will be done by examining nodules in genotypes in which the frequency and/or distribution of exchange events are altered (recombination-defective meiotic mutants, heterozygous structural aberrations). Preliminary data from 2 such mutants suggest that the numbers and locations of exchange events and recombination nodules are altered in parallel. A second objective is to further delineate the role of the wild-type alleles of loci identified by meiotic mutants in insuring a normal meiosis. I have performed serial reconstructions of wild-type oocyte nuclei and have found several features that may be affected by such mutants and therefore provide insights into the process of recombination and into the integration of meiotic processes. These include (a) chromocentral organization of pachytene bivalents; (b) differentiation of synaptonemal complex in euchromatin vs heterochromatin; (c) stage-dependent variation in the length and thickness of the synaptonemal complex; and (d) recombination nodules. The mutants will be examined for these, and other aspects. A number of the functions defined by the recombination-defective mutants are also expressed in somatic tissues; here, the primary role appears to be DNA maintenance and repair. A subsidiary project is the determination of the number and nature of the pathways in both tissues by analysis of double-mutant combinations.